A two-qubit spin–spin entanglement in such a system is quantified in terms of the concurrence [1]}
\(C= \textrm {max}\lbrace 0,\frac{e^{4\beta J}-3}{1+e^{4\beta J\xi }+e^{-4\beta J\xi }+e^{4\beta J}}\rbrace .\) 
